It all depends on how much money you are willing to spend. of course, getting proper 17s will give you the best feeling and will be most fun for staying strictly on the road. however, it tends to be quite pricey.
another route you can go is to get some good dual purpose tires. this also depends on the type of off road you do. if you are mostly doing trails/ fire roads/ logging trails I find DS tires hold up fine and you can still get decent mileage out of them on pavement, as well as good road feedback. Im currently on heidenau scouts, work great on the road and in the wet. they're supposed to get to at least 8,000 road miles, and don't handle too bad off road. air them down all the way, and try to stay away from mud and slippery/wet grass and it's good!
but then, if I had the cash I would go for a set of 17s. if you want to go the cheaper/easier route, you can get 17s with the smaller front rotor, so that you don't have to switch caliper and stuff. or you have to upgrade to the bigger rotor on the 21" wheel. I think it would be too much pain in the a** if you also had to swap out the whole brake assembly every time to switch wheels.